1. Field of the Invention
The present invention relates to a process for the selective agglomeration of sub-bituminous coal fines.
Coal fines, usually occurring as aqueous slurries, are an unavoidable product of conventional coal mining processes and may constitute up to 30% of the mined coal. Such fines normally comprise a mixture of coal-rich and inorganic (mineral matter-rich) particles.
Selective agglomeration methods, which rely on the hydrophobic properties exhibited by coal, may be applied to preferentially collect and agglomerate the coal-rich particles in an oil phase. The coal-rich particles become wetted with an oil layer and adhere to form agglomerates. The hydrophilic inorganic (mineral matter-rich) particles remain unagglomerated and suspended in the aqueous phase. Thus, selective agglomeration involves the addition and dispersion of an immiscible agglomeration agent to the aqueous slurry. Exemplary agglomeration agents would exhibit coal-wetting properties and include hydrocarbon oils or the like. The non-hydrophobic matter is separated from the formed agglomerates by mechanical separation techniques such as screening.
2. Prior Art
The selective agglomeration of bituminous high rank coal fines utilizing high quality oil is well documented, as shown, for example in the teachings of U.S. Pat. Nos. 4,209,301 and 4,153,419. Bituminous high rank coal is generally defined as coal of low oxygen content, (3-14%) and having a carbon content ranging from 79-92%. By high quality oil is meant a low viscosity oil, of low contaminant and low heteroatom content. Chemical affinity permits wetting of the high rank coal surface by the high quality oil to thereby form mechanically strong coal agglomerates.
However, agglomeration of high rank coal utilizing low quality oil, that is oil having a high heteroatom content, results in poor selectivity of the agglomeration process (the concentration of mineral matter in the agglomerates is quite high).
Further, unlike high rank coal, low rank sub-bituminous coals do not form mechanically stable agglomerates, but rather low strength flocs, when high quality distillable oil is utilized as the agglomeration agent. Low rank coals are defined as coals having a carbon content ranging from 78-74%, and a relatively high oxygen content (16-25%).
The characteristics of high rank bituminous and sub-bituminous coals are defined as follows.
______________________________________ High Rank Bituminous Sub-Bituminous ______________________________________ % Moisture 1-10 10-30 % Ash (dry) 3-50 12-40 % VM (daf) 7-38 &gt;38 % Fixed Carbon (daf) 93-62 &lt;62 % O.sub.COOH below 1% 1-10% % C (daf) 79-92 74-78 % H (daf) 4-6.5 3.5-5.5 % O (daf) 3-14 16-25 ______________________________________
The characteristics of the high and low quality oils mentioned can be defined as follows.
______________________________________ High Quality Oils Low Quality Oils ______________________________________ Specif. Gravity (20.degree. C.) 0.8000-0.8500 0.9000-1.1000 Sulfur (%) below 0.05 below 5.0 Total Solids (mg/l) .congruent.1 1-15 Viscosity (CSt at 40.degree. C.) 1-2 3-500 Distillation 100% distillable marginally distillable ______________________________________